Measuring devices require an energy supply for the measuring, evaluation and, in appropriate cases, also display of the measured values. It is, however, not always possible to provide a suitable current connection e.g. via cable. A typical problem arises e.g. when fill levels of liquids need to be measured in mobile units. In such cases, it makes sense to use e.g. batteries or fuel cells. However, if measurements are done in explosion-endangered zones or areas, then special attention to safety is required. For instance, a short circuit might lead to voltages which are too high, and high currents might lead to excessive heating. A further problem is usually that energy units such as batteries or fuel cells are not permitted to be replaced in explosion-endangered areas, because, most often, the process of replacement cancels the protection against explosion or makes it no longer secure.